1. Field of the Invention
The present invention relates to a high frequency circuit module and, more specifically, to a high frequency circuit module in which a high frequency circuit part such as a monolithic microwave integrated circuit (hereinbelow, called an MMIC) and an antenna are provided on the surface and the rear face, respectively, of a multilayer dielectric substrate. More particularly, the invention relates to a high frequency circuit module suitable for an automotive radar module using millimeter waves.
2. Description of the Related Arts
As the most effective system of an intelligent transport system (ITS) solving a traffic accident, traffic jam, environmental problems of exhaust gas, noise, and so one, resource problems due to large consumption of oil energy, and the like caused by “vehicles”, a millimeter wave radar has been developed. In order to equip vehicles as many as possible with millimeter wave radars, realization of an automotive radar module having improved flexibility of a vehicle mounting layout by reducing the size and thickness of the millimeter wave radar, reliability, and low cost is demanded.
As a high frequency circuit module adapted to the automotive radar, a high frequency circuit module in which an antenna and an MMIC are provided on the surface and rear face, respectively, of a multilayer dielectric substrate having therein metallic layers is known.
For example, as shown in FIG. 10 (conventional technique 1), on the surface and rear face of a ceramic multilayer substrate 38 in which a plurality of metallic layers 30 to 33 are provided, an antenna 28 and an MMIC 29 are provided, respectively. As high frequency transmission lines between the antenna 28 and the MMIC 29, microstrip lines 34 and 35 and electro-magnetic coupling slots 36 and 37 are used. Techniques using electro-magnetic coupling slots of this kind are disclosed in Japanese Unexamined Patent Application Nos. 9-237867 and 8-250913. In this example of mounting, when a slot having the same structure is formed over a slot to make the transmission line length shortest, a microstrip line having a length of around λ/2 remains between the slots and works as a resonator. However, when electromagnetic coupling slots are provided above and below the microstrip line, a potential difference occurs between the upper and lower slot metallic layers. Consequently, an electromagnetic wave which propagates in parallel between the slot metallic layers is generated. An amount corresponding to the energy of the electromagnetic wave becomes a loss, so that it difficult to realize the transmission line of a low loss. Therefore, by setting the distance between the electromagnetic coupling slots to λ/2 or longer, interference between the slots is prevented and a loss in the transfer line is minimized. Due to such a structure, the mounting method using the electro-magnetic coupling slots needs a mounting area having the distance of 2λ or longer between the slots, and layout of the upper and lower electronic parts has to be considered so as not to cause interference with the transfer mode of the slot coupling part.
As shown in FIG. 11A (conventional technique 2), there is a known technique in which connection between a plurality of conductive layers 31 and 33 in the multilayer dielectric substrate 38 having the plurality of conductive layers 30 to 33 and 39 is realized by a via satisfying the condition of (R·r)/(2·h)≦L≦(5·R·r)/h (where R, r, and L denote sizes shown in FIG. 11C and h denotes the distance between the conductive layers). When a signal to be transmitted is in a millimeter wave band, the connection between conductive layers in the multilayer substrate formed by the via satisfying the condition can be made by a connecting method of a low loss only in the case where there is one grounding layer connected to the via. However, occurrence of an electromagnetic wave propagating between a plurality of grounding layers cannot be suppressed. Consequently, the method cannot be used to connect the conductors to realize a low loss in the millimeter wave band.
Further, as a technique which does not use a dielectric multilayer substrate, as shown in FIG. 12 (conventional technique 3), there is a technique in which an MMIC 43 and an antenna 44 are provided on the surface and rear face, respectively, of a metal base plate 42, and a coaxial structure 45 formed in the base plate 42 is used to connection the MMIC 43 and antenna 44. In the structure, the RF circuit substrate including the MMIC 43 and the antenna are connected to each other via the coaxial structure, so that a thin, small millimeter wave radar can be relatively easily produced. In the diagram, reference numerals 46, 47, 48, 49, 50 and 51 denote a circuit substrate, an insulating material, an outer terminal, an insulating material, a bonding wire, and a transmission/reception circuit cover, respectively.
As described above, the conventional techniques have problems with respect to easiness in manufacture, manufacturing cost, and circuit characteristics. Particularly, to use the modules for an automotive millimeter wave radar, since the millimeter wave radar is a device mounted outside of a vehicle and use environments of temperature, moisture, vibration, and the like are hostile, generally, an RF circuit has a hermetic structure of interrupting the outside air. Since the transmission loss in the millimeter wave band is much larger as compared with that in a microwave band, the transmission line has to be designed to be as short as possible. Although the line length can be shortened by mounting the RF circuit part on the same face of the substrate as the antenna, it is difficult to mount the RF circuit part and the antenna on the same face due to the limited size of the RF circuit part and the hermetic structure.
In order to mount the RF circuit part and the antenna of the millimeter wave radar as close as possible, the RF circuit part and the antenna are mounted on both sides of the mounting substrate so as to be overlapped, and an oscillator and an amplifier of the RF circuit parts have to be disposed so that the transmission line length becomes the shortest. However, as the mounting substrate of the millimeter wave band, a thin substrate having a dielectric thickness of 0.2 mm or less is used to suppress a transmission line radiation loss. Therefore, the base plate 42 for assuring the mechanical strength is needed as shown in FIG. 12 for the millimeter wave radar. Consequently, the structure whose assembling and processing cost is high has to be employed.
A both-sided two-layer substrate is generally used to assure the characteristics of the millimeter wave transmission line for an RF circuit. A transmission line for a millimeter signal, a power providing line, and a transmission line for a low frequency signal are formed on the same face. Since the high/low frequency signal transmission lines and the power providing line cannot cross each other, aerial wiring such as a bonding wire is required. The higher the frequency of a signal is, the more the signal easily radiates, and it causes a crosstalk in another line. It makes the millimeter waver radar unstable. In addition, since the flexibility of designing of layout of the RF circuit is regulated in the two-layer substrate, reduction in cost by reducing the substrate area of the expensive RF circuit part is limited.